The regulation of body fluid volume and its composition is essential for human life. Several diseases (particularly of the heart, liver, and kidney) disturb volume balance and are associated with an expansion of fluid contained in the extracellular and vascular spaces of the body. An expanded volume state often (but not always) manifests as clinical symptoms of breathlessness (dyspnea) and swelling (edema). Symptoms of congestion lead to millions of hospitalizations worldwide in patients with heart failure (HF) and are associated with significantly worse patient outcomes. Thus, continuous monitoring and optimization of volume status using ambulatory devices may allow reduction of patient morbidity and mortality as well as healthcare utilization and cost.
Ambulatory medical devices include implantable medical devices (IMDs), wearable medical devices, and handheld medical devices. Some examples of IMDs include cardiac function management (CFM) devices such as implantable pacemakers, implantable cardioverter defibrillators (ICDs), subcutaneous implantable cardioverter defibrillators (S-ICDs), cardiac resynchronization therapy devices (CRTs), and devices that include a combination of such capabilities. Other examples of IMDs include implantable drug delivery systems, implantable devices with neural stimulation capability (e.g., vagus nerve stimulator, carotid sinus stimulator, spinal cord stimulator, deep brain stimulator, etc.), and cardiac assist devices. These devices are used to treat patients using electrical or other therapy, or to aid a physician or caregiver in patient diagnosis through internal monitoring of a patient's condition.
Some implantable medical devices can be diagnostic-only devices, such as implantable loop recorders (ILRs) and subcutaneously implantable heart failure monitors (SubQ HFMs). Subcutaneously implantable devices may include a variety of different sensors to monitor one or more internal patient parameters such as electrodes that are able to sense cardiac signals without being in direct contact with the patient's heart, body impedance to detect fluid, motion sensors to sense acceleration and cardiac vibrations, acoustic sensors to measure tissue properties, thermal sensors, and chemosensors to sense the biochemical composition of body fluids.
Some examples of wearable medical devices include wearable cardioverter defibrillators (WCDs) and wearable diagnostic devices (e.g., an ambulatory monitoring vest). WCDs can be monitoring devices that include surface electrodes. The surface electrodes may be arranged to provide one or both of monitoring to provide surface electrocardiograms (ECGs) and delivery of cardioverter and defibrillator shock therapy. A wearable medical device can also include a monitoring patch worn by the patient such as an adherable patch or can be included with an article of clothing worn by the patient.
Monitoring volume status of patients can be challenging. Radioisotope indicator dilution is the gold standard for volume measurement, but is is not feasible for routine clinical or ambulatory monitoring. Methods in clinical use include a variety of biomarkers derived from physical examination (for edema, jugular venous distension, auscultation, orthostatic vital signs), chest radiography, echocardiography, and blood chemistry. While some measures such as physical assessments and chest radiography are not sensitive or specific to volume status, others are not feasible for repeated serial measurements needed for therapy adjustment. Implanted diagnostic-only devices that measure left atrial pressure (LAP) or pulmonary artery pressure (PAP) have been recently available for HF patients.
However these devices require a dedicated implant in the heart or circulatory system of the patient exposing the patient to greater risk of adverse events. A more desirable alternative would be the measurement of a volume status indicator that employ less invasive sensors, or sensors already available in implanted CFM devices. Due to individual sensor limitations, it has not been possible to derive a reliable volume index to date that can be used to optimize patient therapy. A volume index derived from multiple sensor measurements such as heart sounds, impedance, systolic time intervals and respiration with adjustments for patient age, body mass index, cardiac disease and comorbidities overcomes limitations associated with any one sensor or methodology.